U.S. Pat. No. 4,294,902 discloses a typical example of such an image forming method utilizing light-transmissive particles. The principle of the method will be roughly explained in the following. After electrostatic adhesion of light-transmissive particles onto a photo-conductive substrate, when an image is exposed, the electrostatic attraction force between the light-transmissive particles and the substrate is weakened or lost. Accordingly, the difference in electrostatic attraction force between the light-transmissive particles through which light is transmitted and the particles through which no light is transmitted, is utilized for development, whereby a particle image can be obtained.
By this method a full-color image can be obtained through one-time exposure and one-time development. Further, if sublimable color-formers used as image coloring materials are carried by particles, and are sublimated after development onto an image receiving sheet, color-mixing can be effected in their molecular states, and therefore, color-images which are excellent in color-reproducibility can be obtained.
The above-described method, however, produces the following problems.
As is shown in FIG. 1, light-transmissive particles 1 are first spread onto a charged photoconductive substrate 2. The particles 1, which are electroconductive, are electrostatically induced and adhered to the surface of the substrate 2. Ordinarily only one particle layer is formed in order to allow its color separation function to be fully exhibited. Since the light-transmissive particle is spherical or amorphous, gaps are formed among the particles even in the closest packing. In the case of one-layer particles, the covering rate of the particles in relation to the substrate surface is ordinarily 70 to 80%. The gap between the particles becomes a component of electrostatic capacity and is a cause for degradation of color separation ability in the later development stage.
On the electroconductive particles sprayed into one layer opposite charges are stored, the amount of which is large enough to cancel the charges on the entire surface of the charged substrate, or larger, depending on the method of formation of the layer. For example, if a sensitized plate of zinc oxide which is charged to -400 V is used as the substrate 2, the electric potential of the particles after they are formed into one layer is 0.about.+100 V.
An image is next exposed, as is shown in FIG. 2. The reference numeral 1a represents a particle through which no light is transmitted, and 1b a particle through which light is transmitted. Since the substrate portions under the particles through which light is transmitted becomes electroconductive, the charges of the particles escape to the earth, whereby the electric potential of the particles is lowered to approximately 0 V. However, with respect to the particles through which no light is transmitted, the charges are only lost at the substrate portions which are not shaded by the particles, while the portions under the particles are insulated with the charges held. Accordingly the electrostatic capacity between the particles and the substrate becomes smaller than before the exposure (FIG. 2). Since the amount of the charges of the particles at this time is the same as that before the exposure, the potential of the particles through which no light is transmitted rises, as is obvious from the formula Q=CV.
As is described above, since the electrostatic capacity components also exist between the particles on the substrate, if a difference in potential arouses between the particles 1a and 1b, charges are induced into the particles 1b, whereby a new electrostatic attractive force is generated between the particles 1a and 1b. In order to remove only those particles 1b which are in this state, much force is required and particularly at the portions where the color density is thick, namely at the portions which have many particles 1a through which no light is transmitted, it is difficult to remove the particles 1b through which light is transmitted. In this way, the conventional method of developing immediately after exposure is defective in that color purity is lowered at the portions where color density is thick.